Can a measured CO2 concentration show a building is SARS-CoV-2 safe?
An indoor CO2 concentration is commonly used as an indicator of the ventilation rate in indoor spaces, albeit with great uncertainty. Its concentration depends on the number, demographics, and activities of occupants, the outdoor air supply rate, mixing, inter-zonal airflow, the air renewal rate, and the outdoor concentration.
When using CO2 as an indicator, a steady-state concentration must be achieved. Consequently, measurements should typically be made over at least 1 hour to ensure a representative reading. The placement, mixing of air within a space, and accuracy of sensors must be accounted for when analyzing measured data.
The rate at which people shed the virus depends, among other factors, on respiratory activity, such as speaking, singing, coughing, and sneezing, physical activity, and demographics. The dose is a function of occupant physical activity, airborne aerosols concentration, and exposure time.
A CO2 concentration does not indicate with much certainty that the occupants of a building are safe from airborne exposure to the SARS-CoV-2 virus and is not recommended as a reliable proxy of the risk of airborne exposure to the virus. This is particularly true in spaces where there is filtration and UV-C radiation because they eliminate the virus but not the CO2.
In some circumstances a CO2 concentration can provide a crude indication that a building is not adequately ventilated and consequently occupants are likely to be at a higher risk of exposure to the virus. Limiting CO2 concentrations should be determined based on the space and its use, and the outdoor concentration. Occupant activities, such as singing, loud speech, and exercise, increase the emission of aerosols containing the virus. In these circumstances, higher than normal ventilation rates are advised. However, as a rule-of-thumb, if the CO2 concentration is >1400 ppm the building may be considered to be under ventilated.
There are circumstances where a CO2 concentration cannot be used as a proxy for under ventilation. For example, in low occupancy or large volume spaces where there is much less certainty in the relationship between CO2 concentration and the ventilation rate.
Finally, CO2 should not be used when its source is not exclusively from people, such as combustion devices.
Benjamin Jones, University of Nottingham, Jelle Laverge, Ghent University, Pawel Wargocki, Technical University of Denmark
- AIVC. AIVC Newsletter Special Issue on COVID-19. November 2020